1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to a synthetic method for the preparation of an improved cathode active material for nonaqueous electrolyte alkali metal electrochemical cells. The present synthetic method provides a cathode material which predominantly contains a single phase mixed metal oxide, such as a single phase silver vanadium oxide (SVO, Ag2V4O11). An example of a discharge application using this material is an implantable cardiac defibrillator where a lithium electrochemical cell may run under a light load for extended periods of time interrupted from time to time by high rate pulse discharge. The lithium electrochemical cell having a cathode of a single phase active material provides improved discharge characteristics including energy density and pulse discharge energy with reduced voltage delay in comparison to a mixed phase mixed metal oxide cathode active materials.
2. Prior Art
U.S. Pat. No. 4,310,609 to Liang et al. and U.S. Pat. No. 4,391,729 to Liang et al., which are assigned to the assignee of the present invention and incorporate herein by reference, disclose the preparation of a mixed phase mixed metal oxide cathode active material, such as a mixed phase silver vanadium oxide, by a thermal decomposition reaction for use in a nonaqueous electrolyte alkali metal electrochemical cell. The thermal decomposition reaction begins with an initial heating of starting materials at 180xc2x0 C. followed by a second heat treatment step at 360xc2x0 C. Notably, the 180xc2x0 C. step is only held long enough to bring the starting materials to thermal equilibrium, after which the sample is further heated directly to 360xc2x0 C. The product mixed metal oxide cathode active material has been shown to have multiple phases. For example, when silver vanadium oxide is the intended active product phases other than the desired xcex5-phase (Ag2V4O11), including xcex3-phase (Ag0.74V2O5.37) and xcex2 phase (Ag0.35V2O5.18) as well as unreacted starting materials, are produced.
U.S. Pat. No. 5,695,892 to Leising et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes the synthesis of a mixed phase mixed metal oxide, such as silver vanadium oxide, under an air atmosphere. When silver vanadium oxide is the intended product, the thermal synthesis profile consists of heating starting materials to 110xc2x0 C. for several hours, followed by heating to 230xc2x0 C. for 30 minutes, then 260xc2x0 C. for two hours, then 300xc2x0 C. for 16 hours, and finally 375xc2x0 C. for at least 24 hours. This material displays multiple peaks in the differential scanning calorimetry (DSC) curve at 459xc2x0 C., 533xc2x0 C., and 544xc2x0 C. due to the presence of multiple phases in the final product.
Thus, the various prior art synthesis techniques result in more than one reaction mechanism taking place or they result in incomplete reaction of the starting materials. When silver vanadium oxide is the intended product, in addition to the desired xcex5-phase, xcex3-phase and xcex2-phase active materials and the unreacted starting material of AgVO3 are also known to be produced in varying quantities.
The publication xe2x80x9cLithium/Silver Vanadium Oxide Batteries with Various Silver to Vanadium Ratiosxe2x80x9d, Takeuchi, E. S.; Piliero, P., J. Power Sources, 1987, 21, 133-141, details the discharge of lithium batteries containing silver vanadium oxide with differing ratios of silver to vanadium. This publication describes that preparation ratios of Ag:V of 0.76:2 (low silver) and 1:1 (high silver) give less capacity and distinctly different discharge voltage profiles than silver vanadium oxide prepared with a Ag to V ratio of 1:2. Thus, the delivered capacity and voltage profile of a lithium/silver vanadium oxide cell is dictated by the phases of silver vanadium oxide making up the cathode active material. A similar conclusion is reached for other mixed metal oxide active materials produced in thermal decomposition reactions. In other words, the phase make up of a silver vanadium oxide active material greatly effects the delivered capacity and the voltage profile of the resulting lithium electrochemical cell. With this in mind, a highly consistent single phase mixed metal oxide material and, in particular, a single phase silver vanadium oxide is desired as a cathode active material for lithium electrochemical cells so that the discharge properties of the cell may be predicted in a reliable fashion. Accordingly, the present invention is directed to the synthesis of single phase mixed metal oxide materials using a new synthesis technique.
The present invention is directed to a process for preparing a cathode active material consisting of a single phase mixed metal oxide, such as a single phase silver vanadium oxide. The synthesis technique involves first heating the starting materials to melt a decomposable starting constituent. This first heating temperature is held for a period of time sufficient to enable the decomposable starting constituent to melt and completely flow throughout and within the other starting materials. Then, the thusly produced melt impregnated reaction admixture is preferably ground to ensure complete homogeneity of the starting materials, followed by heating to the decomposition temperature of the decomposable starting constituent. To finish the synthesis, the decomposed admixture is heated to an elevated temperature above the decomposition temperature to provide the single phase mixed metal oxide.
When the thusly prepared cathode active material is coupled with a lithium anode, the predictability of the cell""s discharge performance is greatly improved over that of prior art primary lithium cells containing mixed metal oxides of multiple phases and of unreacted starting materials. Silver vanadium oxide cathode material prepared by the present melt impregnation technique has been shown to be a single phase mixed metal oxide product material.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and to the appended drawings.